Clark County contracted with Otak to update the County's stormwater ordinance. As part of this work Clear Creek Solutions was hired to produce HSPF calibrations of two watersheds in the county. The calibrated HSPF parameter values were then used to update the Western Washington Hydrology Model (WWHM) for Clark County.

In order to employ HSPF as the hydrologic stormwater engine for WWHM, it must be calibrated to watersheds that reflect the range of climatic, topographic, soil, and land use conditions within Clark County. In this study, model simulations were performed for the Mill Creek and Gee Creek watersheds. Topographic, soils, and land use/cover information was used to develop the model segmentation and parameter inputs for the watershed models. Both quantitative and qualitative comparisons were performed to support the model performance evaluation effort.

Based on the model results, the HSPF application to the Mill Creek and Gee Creek watersheds provide a sound, calibrated and validated HSPF parameter values for Clark County. The resulting model parameter values are appropriate for use in WWHM, and for impact evaluation of flood control alternatives to meet the Washington State Department of Ecology's requirements. The calibration results, based on the weight-of-evidence approach described herein, demonstrate a good representation of the observed data. This is the outcome of a wide range of graphical comparisons and measures of model performance for flow duration and individual storm event simulations. These comparisons demonstrate conclusively that the models are a good representation of the water balance and hydrology of the watersheds.

New HSPF pervious land segments (PERLNDs) and calibrated parameter values replaced the standard PERLNDs and values used in WWHM for Clark County in support of the County's updated stormwater ordinance.